In the United States, opioid use disorder (OUD) and overdose deaths are increasing; yet 80% of people with OUD remain out of treatment. Operating in 33 states, syringe exchange programs (SEPs), which provide sterile syringes and other health services to people who inject drugs, are a key venue to reach out-of-treatment opioid users. In a 24-week randomized controlled trial based in a large urban area with high rates of OUD and HIV, Project BLOOM (R01DA044878, [PI: Fox]) will recruit 250 out-of-treatment opioid users who utilize SEPs and randomize 1:1 to onsite buprenorphine treatment (O-BMT) or enhanced referral. Over two weeks, participants in the O-BMT condition will see a buprenorphine provider twice onsite, receive weekly medication packs, and then their care will be transferred to a community health center for maintenance buprenorphine treatment. The proposed diversity supplement will support Dr. Teresa Lpez-Castro, a clinical psychologist and early-stage investigator, to study barriers to engagement in buprenorphine treatment that are related to Post- Traumatic Stress Disorder (PTSD) and traumatic event re-exposure. Project BLOOM has identified structural barriers to OUD treatment and is testing SEP-based buprenorphine initiation as a way to minimize delays in starting buprenorphine treatment. However, PTSD and the experiencing of new traumatic events (traumatic event re-exposure) are exceedingly common among people with OUD, affect drug use behaviors, and will likely modify O-BMT?s effects. Thus designing novel engagement and treatment strategies requires consideration of PTSD and traumatic event re-exposure. The objectives of the proposed mixed methods study are two-fold: (1) to quantitatively evaluate the moderating effect of PTSD and traumatic event re-exposure on the efficacy of O-BMT; and (2) to collect qualitative data from stakeholders (SEP consumers and staff) on the PTSD-related treatment needs for IDUs. The proposed two-year diversity supplement will create generalizable knowledge regarding how PTSD and traumatic event re- exposure impact buprenorphine treatment efficacy. The application also proposes a comprehensive career development plan that will also support Dr. Lpez-Castro?s further development as a clinical investigator.